Linking deep-seated tectonic processes to surface mineralization systems is a fundamental challenge in earth sciences. The Early Jurassic Laguocuo ophiolitic mélange in central Tibet, situated adjacent to a major Li-rich brine basin, provides a natural laboratory to decipher the complete pathway of lithium from a mantle source to an ore deposit. Through an integrated investigation of field petrology, zircon U-Pb-Hf isotopes, and whole-rock geochemistry, we unravel this deep-to-surface connection. The plagiogranites, crystallized at 183.6 ± 1.9 Ma from a depleted mantle-derived magma, record a complex magmatic evolution marked by disequilibrium mineral phases. Their unique geochemical signatures—including flat REE patterns, significant LILE enrichment, and Nb-Ta-Ti depletion, combined with high positive εHf(t) values—collectively indicate that the mélange represents a fossil back-arc basin formed within an ocean–continent subduction system. Critically, these plagiogranites are systematically depleted in highly incompatible elements like Li and B, a depletion that is starkly mirrored by the enrichment of these same elements in the adjacent Laguocuo brines. This complementary relationship is the key to deciphering the lithium pathway. We, therefore, propose a coupled model where Early Jurassic back-arc extension drove magmatic differentiation, and subsequent exsolution of late-stage, Li-rich hydrothermal fluids from the crystallizing pluton transported the ore-forming components to the surface basin. Our study successfully deciphers this lithium pathway, demonstrating a direct genetic link between a specific deep tectonic setting (back-arc basin) and the formation of a supergene lithium brine deposit, with the ophiolitic mélange acting as the crucial geological archive.